Method for obtaining smooth winding surfaces on rough miniature cores



April 26, 1966 P. R. MUNK 3, ,5 METHOD FOR OBTAINING SMOOTH WINDINGSURFACES ON ROUGH MINIATURE CORES Filed May 17, 1962 mwmm'mw IN VE N TORI P. R. MUNK ATTORNEY United States Patent 3,247,572 METHOD FOROBTAINING SMOOTH WINDING SURFACES ON ROUGH MINIATURE CORES Paul R. Munk,Madison, N.J., assignor to Bell Telephone Laboratories, Incorporated,New York, N.Y., a corporation of New York Filed May 17, 1962, Ser. No.196,014 6 Claims. ((31. 29-1555) This invention relates to a process forobtaining smooth surfaces on an initially rough miniature object andmore specifically to a manufacturing process for obtaining a smoothwinding surface on miniature magnetic cores for microcircuits.

In order to obtain the physical and magnetic properties desired inminiature magnetic devices, materials, such as ferrite, are used tocompose the core. Because these materials are generally hard in nature,the existing manufacturing processes, used to form the material into thecore shape, leave sharp and ragged edges. These edges must be removed orcovered before the windings are applied to the core or the sharp edgeswill scrape the insulation from the wires as it is wound on the core,resulting in turn-to-turn short circuits, erratic performance of themagnetic device or possible failure of a system.

In devices wherein the diameter or length measurements ofthe core areequal to or greater than approximately one-quarter inch, it has beenfound that satisfactory winding surfaces maybe obtained byproviding asmooth disc'or core-shaped mask that can be applied to the core to coverthe sharp and ragged edges left by the manufacturing core-formingprocess. Numerous methods besides the one listed above find applicationwhen typical core dimensions are in excess of one-quarter inch.

When typical core dimensions are considerably less than one-quarterinch, e.g., 0.040 inch, truly miniature cores, such methods as taping,lacquering', providing auxiliary masks, tumbling or sandblasting or likeprocesses, are economically unfeasible. Such methods besides being tooexpensive and slow, change the physical dimensions of the core; therebyobjectionably reducing the winding space or altering the magneticcharacteristics of the core. It is pointed out to further explain theapplication of this invention that as the core dimensions get smaller,roughness and irregularities, such as sharp edges, get larger inproportion to the core size. Loosely speaking, microscopicirregularities may become of major importance if core dimensions arereduced enough.

An object of the invention is to provide a manufacturing process forobtaining smooth surfaces on miniature objects or bodies.

A feature of the invention is the provision of a manufacturing processwherein areas of an object are automatically selected and subjected toan eroding process.

Another feature of the invention is the provision of a manufacturingprocess wherein the shortcomings of a liquid coating process are used toadvantage in a subsequent eroding process.

A further feature of the invention is the provision of a manufacturingprocess wherein the characteristics of a liquid insulating material areused to complementa subsequent portion of the process.

As discussed above, the invention finds particular application in themanufacturing and finishing of miniature cores for microcircuits. It hasbeen known to cover magnetic cores with successive coatings of lacquer,varnish or other like insulating materials until sharp edges arecovered. These successive coatings have a tendency to collect on broadareas and in holes distorting the desired shape and increasing the wirewinding difficulties because of the decrease in the dimensions of coreholes due to the lacquer buildup. It has also been known to erode orabrade the sharp edges and corners from the core by 3,247,572 PatentedApr. 26, 1966 sandblasting, tumbling or other such processes. Theseprocesses have a tendency to erode broad areas or surfaces in preferenceto sharp edges. It has been found that after suflicient eroding toremove sharp or ragged edges, the core dimensions have been sufiicientlyaltered, due to the erosion of the broad areas, to change the desiredmag netic qualities. 7 I

The proposed invention includes a novel combination of the two aboveprocesses wherein the disadvantages of the liquid coating process areaccented and taken advantage of in the eroding process. The proposedprocess comprises the application to a miniature object, such as amagnetic core, of a highly diluted liquid, electrically insulatingmaterial. The liquid insulating material is highly diluted to accent thesurface tension effect of the liquid by reducing the viscosity of thematerial. Such reduction of viscosity encourages the liquid material topull away from and expose sharp or ragged corners and edges. Theresulting liquid coating is cured to a resilient condition so that it isneither tacky nor brittle. The prepared core is then subjected to aneroding process such as sandblasting, shot-peening or the like. Due tothe selective covering and the resilient condition of the coating, thesharp edges and corners are exposed to the eroding process and the broadareas are protected from the same by the coating. After rounding of thesharp edges and corners by the eroding process, a second application ofa liquid electrically insulating material is applied to the core whichcompletely envelops the rounded core and provides a smooth electricallyinsulating coating; Upon a second curing process wherein the secondliquid coating is completely dried, the miniature core is ready for thewire winding operation.

The invention will be more clearly understood from the followingdetailed description, when read in conjunction with the drawing, inwhich:

FIG. 1 is an isometric view of a lattice 0r ladder type magnetic core;

FIG. 2 is an isometric view of a circular core indicating in anexaggerated manner two ragged or sharp edges left by the core-formingprocess; 7

FIG. 3 is a sectioned isometric view of a circular core after subjectionto my process;

FIG. 4 is an enlarged view of a portion of a circular core after thefirst coating process but before the erosion process.

The lattice core shown in FIG. 1 is presented to exemplify the field towhich my process finds greatest application. Typical thicknessdimensions of the Webs 10 and the stringers 11 are approximately 0.040inch. Typical dimensions of the rectangular openings of thelattice areapproximately 0.040 by 0.060 inch. The entire length of such astructure, depending on how many windings and webs are needed,'can bemuch less than one inch.

A circular core 12 is included in each of the FIGS. 2, 3, and 4 toexplain the proposed process. FIG. 2 sets forth a circular core with tworough edges 13 and 14 that are left from the core-forming process.Removal of these edges 13 and 14 is mandatory for proper performance ofthe magnetic circuit.

My process comprises the application to the core by such means asbrushing, spraying or dipping of a first coating 15 of liquid insulatingmaterial such as lacquer,

' varnish, or other similar materials as SllOIWIl in FIG. 4.

brittle nor tacky. Such a physical condition of the coating 15 isnecessary for the eroding operation that follows. If

The coated core is then cured or dried until the coating 15 has aresilient condition and is neither the coating 15 is brittle, theeroding process will chip away the coating, exposing the broad surfaces16 to the eroding process and if the coating is tacky, particles used inthe eroding process will adhere to the coating, causing a detrimentalbuildup.

The prepared core, as shown in FIG. 4, is then subjected to an erodingprocess such. as sandblasting, shot-peeuing, tumbling and the like,which removes the exposed sharp edges 13 and 14 but does not erode thebroad areas 16 due to the automatic selection of the first dilutedliquid coating 15. The disadvantages accented and produced by thedilution of the liquid applied in the first coating 15 produce anautomatic selective exposure and protection to the core 12 which is usedto advantage in the subsequent eroding process.

After the eroding process, a second coating 17 of liquidinsulatingmaterial is applied to the core 12 as shown in FIG. 3. The secondcoating completely envelops the core 12 covering the first coating 15and the rounded edges left by the eroding process and providing a smoothWinding surface. After curing of the second coating 17, the core appearsas shown in FIG. 3.

It is obvious that the invention is not limited to the specificapplication illustrated herein but can be employed by those skilled inthe art in many ways too numerous to mention. Such use is within thescope of the appended claims.

What is claimed is:

1. A method for obtaining smooth surfaces on rough miniature objectshaving broad surfaces and sharp projections comprising applying a liquidmaterial to saidv objects, said material having a surface tension ofappropriate magnitude so that the resulting coating on the objects pullsaway from said sharp projections and collects on said broad surfaces ofsaid objects, curing said material to form a coating on said surfacesthat is neither brittle nor tacky, subjecting the prepared objects toerosion to remove the exposed sharp projecti-ons, and forming from asecond liquid material a uniform electrically insulating coating on saidobjects that covers the broad surfaces and the rounded edges left by theerosion process.

2. A method according to claim 1 wherein said liquid materials areelectrically insulating materials.

3. A method for obtaining smooth surfaces on miniature objects havingrough portions and smooth portions comprising applying a first curableliquid material to said objects, the latter being highly diluted so thatthe resulting coating on said objects pulls away from said roughportions and coats said smooth portions because of the reduced surfacetension of the liquid, curing saidmaterial to form a resilient coatingon said smooth portions, subjecting said objects to an abrading processto remove said rough portions, applying a second curable liquid materialto said objects resulting in a uniform insulating coating that coversthe broad surfaces and the rounded edges left by the abrading processand curing said second liquid material.

4. A method according to claim 3 wherein said first and second liquidmaterials are self-hardening and electrically insulating materials.

5. The method of. selectively reducing rough portions of the surface ofa miniature object Without reduction of the remaining surface thatcomprises applying to the body a coating of liquid materials ofsufticiently low surface tension to recede from and thereby leaveexposed said rough portions while covering the remainder of the body,curing the coating to a resilient, nontacky condition; eroding away theexposed rough portions, applying a protective coating to the body, andcompletely curing the latter coating.

6. The method set forth in claim 1 wherein said second mentioned coatingis cured to at leasta resilient condition.

References Cited by the Examiner- UNITED STATES PATENTS 7/1957 Wofskill51312 X 8/1959 Beckner et 'al. 51-319 X

1. A METHOD FOR OBTAINING SMOOTH SURFACES ON ROUGH MINIATURE OBJECTSHAVING BROAD SURFACES AND SHARP PROJECTIONS COMPRISING APPLYING A LIQUIDMATERIAL TO SAID OBJECTS, SAID MATERIAL HAVING A SURFACE TENSION OFAPPROPRIATE MAGNITUDE SO THAT THE RESULTING COATING ON THE OBJECTS PULLSAWAY FROM SAID SHARP PROJECTIONS AND COLLECTS ON SAID BROAD SURFACES OFSAID OBJECTS, CURING SAID MATERIAL TO FORM A COATING ON SAID SURFACESTHAT IS NEITHER BRITTLE NOR TACKY, SUBJECTING THE PREPARED OBJECTS TOEROSION TO REMOVE THE EXPOSED SHARP PROJECTIONS, AND FORMING FROM ASECOND LIQUID MATERIAL IN UNIFORM ELECTRICALLY INSULATING COATING ONSAID OBJECTS THAT COVERS THE BROAD SURFACES AND THE ROUNDED EDGES LEFTBY THE EROSION PROCESS.